For some background information, see Robotics
I started using 3D CAD software as a high school student in the 2004 FIRST Robotics Competition (FRC). The 2006 challenge involved twelve inch diameter foam balls. The mechanism featured here is a conveyor belt I designed to move the game pieces in and out of a hopper. Loops of silicone tubing wrapped around two rollers. This conveyor pinches the ball against a smooth plate, making the ball roll around its circumference. Other concepts, such as two linear actuators on either side of the ball, would have sped up the travel. However, this one-sided design was chosen for economy of volume and weight (two constraints on FRC Robots).
I personally find hand drawings to be helpful in the initial stages of a design. Paper and pencil (or whiteboard and marker) can be less demanding than Inventor or Solidworks. Collaboration can be more spontaneous and less aggravating than formally sketching in software over a network. This whiteboard drawing is well-developed and indicates the major components of the system. I used it as an outline to guide my parts, drawings, and assemblies in Inventor.
Exploded views are helpful for assembly and maintenance. By stretching out the constraints, it's easier to see how components like machine screws interface to their respective holes. Although time consuming, detailed CAD models can catch interferences before they are constructed, and provide valuable reference material during operation.
However, virtualization is not perfect. Modeling necessarily entails approximations and assumptions. Sometimes there is no substitute for field testing. In competition, the silicone tubes used as conveyor material tended to break at the weld. Fortunately, the tubing was abundant and straightforward to replace. The MkIII Robot seeded 1st in the 2006 Davis-Sacramento Regional.